wrote:
Mike Lepore wrote:
Someone please tell me why spacecraft are designed to reenter
the earth's atmosphere at high speed. Isn't there some way to
come down slowly,
so the heat shields wouldn't be needed? Has anyone modeled
the idea of unfolding some large wings to add a lot of
surface area, or using propellers to resist falling, or
parachutes? Thank you.

A spacecraft enters the atmosphere at such high speed
because its orbital speed is so high to begin with. On orbit a
satellite has a certain (large) orbital speed, and if some of
that speed is lost, for instance by firing its rocket motor
forward, then the shape of the orbit changes so that it dips
downward, closer to Earth. If the satellite loses enough
speed, then the path dips right down deep into the atmosphere.
This is, in fact, how a returning spacecraft is made to lose
enough of its speed to allow it to land safely, by plowing
through the atmosphere.

How much could the Shuttle decelerate with its own engines, given an
unlimited supply of fuel, before it encountered significant
atmospheric drag? I was thinking that if they built a (relatively)
cheap and cheerful rocket for the sole purpose of getting rocket fuel
into orbit, the Shuttle could use that for its descent, and the same
orbiting refueling stations could also be used for spacecraft leaving
Earth orbit, making (say) missions to other planets cheaper and
quicker.

Of course, as far as the Shuttle is concerned, the easiest solution
would be just to make its heat shield 100% reliable...